Polyesters are industrially well known compounds, and various types of polyesters are in use. In particular, polyester polyols can be crosslinked and cured with various crosslinking agents, e.g., isocyanate compounds, and are frequently used in applications such as coating materials, adhesives, inks, and sealants.
Of such polyester polyols, crystalline polyester polyols not only have excellent mechanical properties but have the following property. At temperatures not lower than the melting point thereof, they can be handled as a relatively low-viscosity liquid; upon cooling to a temperature not higher than the crystallization temperature thereof, they solidify in a short time period through recrystallization. Owing to this property, the crystalline polyester polyols are coming to be increasingly utilized as components of reactive hot-melt adhesives, ink-jet inks for hot-melt use, etc.
In particular, use of reactive hot-melt adhesives is rapidly expanding because the adhesives are excellent in strength and bonding rate and highly suitable for use in assembly lines and because they meet the social demand for nonuse of solvents and energy saving. There also is a strong desire for improved suitability for use in continuous operation, and a reactive hot-melt adhesive having a higher setting rate is desired.
It is known that the degree of crystallization of a polyester polyol influences the setting rate thereof; this fact, which can be used for meeting that demand, is disclosed, e.g., in Setchaku, 1984, Vol.28, No.8, p.5 and ADHESIVES AGE, 1987, November issue, p.32. Namely, a polyester polyol having a high degree of crystallization is extremely advantageous for improving setting rate.
Known starting materials for those polyester polyols are as follows. Examples of polycarboxylic acid ingredients include terephthalic acid, isophthalic acid, succinic acid, adipic acid, azelaic acid, sebacic acid, and dodecanedioic acid. Examples of diol ingredients include ethylene glycol, propylene glycol, 1,4-butanediol, 1,5-pentanediol, 1,6-hexanediol, and 1,4-cyclohexanedimethanol. Of the polyester polyols obtained from combinations of these monomers, polyester polyols for use as a material for hot-melt adhesives having an improved setting rate are disclosed in Japanese Patent Laid-Open No. 88686/1990. These polyester polyols include one formed from dodecanedioic acid and 1,6-hexanediol, one formed from sebacic acid and 1,6-hexanediol, and one formed from dodecanedioic acid and ethylene glycol.
The polyester polyol formed from dodecanedioic acid and 1,6-hexanediol, in particular, has a far higher setting rate than in conventional techniques. However, this polyester polyol is used in limited amounts because dodecanedioic acid is expensive.
On the other hand, reactive hot-melt adhesives based on a highly crystalline polyester polyol have problems remaining unsolved, for example, that they have a low rate of moisture absorption and should hence be allowed to stand for a prolonged time period so as to have an increased bonding strength through crosslinking reactions with water.
Japanese Patent Laid-Open No. 304223/1992 discloses a mixture for urethane prepolymer production which is prepared by mixing a polyester polyol obtained using dodecanedioic acid with a polyester polyol obtained using an inexpensive acid ingredient, e.g., adipic acid.
However, the polyester polyol obtained using dodecanedioic acid has poor compatibility with the polyester polyol obtained using adipic acid, so that the mixture prepared by mixing these polyester polyols suffer phase separation. This means that when the polyester polyol mixture is reacted with a polyisocyanate in producing a urethane prepolymer, the reaction proceeds unevenly, resulting in impaired workability.